UNCG has an innovative Learning Assistant (LA) program, in which upper-class undergraduate physics majors teach laboratory sections of the introductory calculus-based physics sequence. The lecture section's professor provides supervision and determines the overall learning objectives and structure of the labs, but the team of LAs develop the detailed lesson plans, write up all handouts and quizzes, conduct the lab sessions, and evaluate student work. This gives the LAs a genuine voice in planning and teaching, and increases the authenticity of the teaching experience. In order to investigate the impact of this teaching experience upon physics majors, we interviewed five current and former LAs. We analyzed the interview transcripts via emergent thematic analysis to identify the most prevalent impacts, and then viewed the results through the lens of professional identity development. We claim that the LA experience helps grow three aspects of physics majors' professional identity: their sense of themselves as a physics teacher, as a physics student, and as a member of a community of practice.